QB 

543 
-78 

U45 


UC-NRLF 


INSTRUCTIONS 


OBSERVING 


TOTAL  SOLAR  ECLIPSE 


2C 


1878. 
/ 


ttENT 


ISSUED   BY  THE/U.   S.  NAVAL  OBSERVATORY 


WASHINGTON: 

GOVERNMENT    PRINTING    OFFICE. 
1878. 


INSTRUCTIONS 


OBSERVING 


THE 


TOTAL  SOLAR  ECLIPSE 


ULY      2(),      10J 


818. 


ISSUED   BY  THE/U.  S.  NAVAL  OBSERVATORY. 

*        ' -_,- 


WASHINGTON: 

GOVERNMENT    PRINTING    OFFICE. 
1878. 


'72 

UNS 


OF   CONTENTS. 


Page 
I.  Observations  with  the  Naked  Eye _ 

II.  To  Photographers 

III.  The  Photoheliograph .!..'!  !o 

IV.  The  Equatorial  Camera 

V.  Telescopic  Observations 

VI.  Spectroscopic  Observations -_  2O 

VII.  Polariscopic  Observations 

VIII.  Photometric  Observations 

IX.  Therino-Electric  Observations 


76<) 


These  instructions  were,  by  my  direction,  written  by  Prof.  William  Harkness, 
United  States  Navy. 

Suctions  I,  II,  V,  and  Vlll  describe  such  observations  as  can  be  made  with  the 
apparatus  usually  possessed  by  amateurs ;  while  the  other  sections  relate  mostly  to 
observations  which  can  only  be  carried  out  by  persons  who  are  able  to  command 
expensive  apparatus,  and  who  are'  skilled  in  astronomy  and  physics. 

JOHN  RODGERS, 
Bear-Admiral,  U.  S.  N.,  Superintendent. 


INSTRUCTIONS  FOR  OBSERVING  THE  TOTAL  SOLAR  ECLIPSE  OF 

.JULY  29,   1878. 


The  following  suggestions  are  intended  to  direct  the  attention  of  persons  who 
may  witness  the  total  solar  eclipse  of  July  29,  and  who  may  desire  to  co-operate 
with  the  United  States  Naval  Observatory,  to  some  of  the  phenomena  which,  in  the 
present  state  of  science,  it  is  most  desirable  should  be  carefully  observed  on  that 
occasion. 

In  writing  these  suggestions,  much  assistance  has  been  derived  from  the  instruc- 
tions issued  to  the  government  eclipse  expeditions  sent  out  by  various  nations  during 
the  last  ten  years  ;  and  free  use  has  also  been  made  of  the  reports  of  these  expeditions. 

SECTION  I -OBSERVATIONS  WITH  THE  NAKED  EYE. 

i.  At  all  places  on  the  central  line  of  the  eclipse,  the  following  phenomena  will 
be  seen  during  totality,  if  the  sky  is  clear,  viz:  Just  as  the  sun's  last  ray  fades 
out,  some  glowing  points  of  light  will  start  into  view,  hanging  upon  the  edge  of  the 
black  moon  and  glistening  like  rubies ;  while  surrounding  the  whole  will  be  seen  a 
halo  whose  mild  radiance  reminds  the  beholder  of  the  glories  with  which  the  painters 
of  old  adorned  the  heads  of  saints.  The  glowing  points  are  the  red  prominences,  and 
the  halo  is  the  corona  ;  both  of  which  are  now  known  to  be  solar  appendages.  The 
corona  consists  of  a  comparatively  bright  inner  part  lying  close  to  the  sun,  surrounded 
by  a  much  fainter  mass  of  luminous  matter  of  vast  extent,  and  generally  of  most 
irregular  form.  Accurate  drawings  of  it  will  be  exceedingly  valuable,  and  for- 
tunately, inaccuracy,  such  as  affects  the  scientific  value  of  the  drawings,  can  be  easily 
avoided.  With  this  view,  persons  intending  to  make  sketches  should  provide  them- 
selves with  a  sheet  of  paper  about  9  inches  wide  by  1 2  inches  long,  having  upon  it  a 
black  disk,  i^  inches  in  diameter,  to  represent  the  moon,  from  the  center  of  which 
straight  lines  are  drawn  at  angles  of  thirty  degrees,  the  whole  being  an  exact  copy  of 
Plate  I.  Let  a  weight  be  suspended  by  a  string  in  such  a  position  that  the  observer 
can  see  it  hanging  over  the  sun's  center ;  and  let  the  diagram  upon  which  the  draw- 
ing is  to  be  made  be  so  placed  upon  any  convenient  stand  that  the  line  marked  "  top  " 
"  bottom"  shall  be  in  the  plane  passing  through  the  observer's  eye,  the  string,  and  the 
sun's  center,  the  end  marked  "top"  corresponding  to  the  top  of  the  string.  Also,  let 
a  lighted  lantern  be  ready  to  illuminate  the  paper,  in  case  the  light  of  the  corona  is 
insufficient.  Finally*,  two  or  three  minutes  before  the  beginning  of  totality,  let  the 
observer  close  his  eyes  and  cover  them  with  a  handkerchief,  so  as  to  make  them  as 
sensitive  as  possible.  A  friend  must  be  ready  to  warn  him  when  totality  begins,  and 
then,  removing  the  handkerchief,  he  must  rapidly  sketch  in  the  outline  of  the  faint 
outer  parts  of  the  corona,  taking  special  care  to  preserve  their  correct  proportions 


s 

relatively  to  the  moon,  and  above  all  being  scrupulously  particular  to  place  the 
prominently  marked  branches  and  rifts  in  their  proper  positions.  In  doing  this, 
-real  assistance  \\ill  lie  afforded  by  the  line-  drawn  upon  the  diagram  from  the  center 
of  the  black  disk.  Having  completed  the  outer  portions  of  the  corona,  the  outline  of 
the  bright  inner  part  should  be  sketched  :  and  in  doing  this  it  will  probably  be  best 
to  view  the  eclipse  through  a  spectacle  glass  of  a  light  green  tint.  Finally,  if  any 
time  remains,  the  whole  drawing  should  be  carefully  revised;  with  the  aid  of  an  opera- 
Lj-lass.  if  possible.*  It  is  not  worth  while  to  insert  the  red  prominences.  The  duration 
of  totality  being  only  three  minutes  in  the  most  favorable  localities,  it  is  useless  to 
attempt  making  more  than  a  single  drawing:  in  fact,  such  an  attempt  would  show 
that  the  work  had  been  so  hurriedly  done  as  to  deprive  it  of  all  value. 

Plate  II  is  a  drawing  of  the  eclipse  of  April  16,  1874,  given  by  .Mr.  K.  .).  Stone, 
director  of  the  Royal  Observatory  at  the  ( 'ape  of  ( iood  Ilope.f  It  shows  the  style  in 
which  the  drawings  should  be  made  :  <t,  «,  ",  being  the  outline  of  the  faint  outer  portion, 
and  b,  l>,  //,  the  outline  of  the  bright  inner  part,  of  the  corona.  Probably  many  parts  of 
the  outer  corona  fade  away  so  gradually  that  it  will  be  impossible  to  say  precisely 
where  they  terminate.  This  uncertainty  should  be  indicated  by  a  serrated  outline, 
as  shown  at  a'.  The  original  sketches  must  not  be  altered  after  the  totality  is  over: 
but  if  the  observer  desires  to  make  a  finished  drawing  from  memory,  it  may  In- 
done  upon  another  piece  of  paper. 

2.  Portions  of  the  corona  may  probably  be  seen  for  a  few  seconds,  both    before 
and  after  the  totality.      Note  the   exact    positions   and    appearances  of  these   portions, 
observing  particularly  whether  the   parts   seen   before   totality  are    the   same  as  those 
seen  after  totality,  and  how  long  they  are  visible  before  and  after  totality. 

3.  Note   the   character  of  the   extreme   outer    boundary  of  the   corona.       hoes   it 
fade  gradually  away,  or  has  it  a  definite  edge  f 

4.  The   moon's   shadow  should  be  bordered   by  diffraction  bands;    and  some  ob- 
s(  rvers  claim  to  have  seen  them  as  dark  shadows -living  over  the  earth's  surface  imme- 
diately  In-fore   and    immediately   after  totality.      Probably    they  would  be  most  \isible 
from  an  eminence  commanding  an  extensive   landscape,    and  it    would    be   a    matter  of 
interest  to  look  for  them. 

5.  As  the  truth  of  Leverrier's  discovery  of   an  apparently  unexplained  motion  of 
the  perihelion  of  Mercury  is  now  established  beyond  all  doubt,  it  is  important  to  renew 
the  search  for  an  intra-Merciirial   planet  or  planets.      With  this   view,  the  parts  of  the 
ecliptic  near  the  sun  should  be  carefully  examined   during   the   totality  of  the   coining 
eclipse,  and  if  an\  thing  hitherto  unknown  is  discovered,  its  bearing  and  distance  from 
the  sun    should    be   accurately  measured.      The   bearing   can    be    defined  by  holding  a 
watch  between  the  eye  and  the  corona,  and   giving  the  hour   which  points  toward  tin- 
object,  it  being  understood  that  the  XII  is  toward  the  zenith.      The  distance  should  be 
li'iveu  in  inches  and  parts  of  an  inch,  as  measured  by  a  rule  held  exactly  eighteen  inches 
from  the  eye       To  insure  the  rule  being  <o  held   a  string  should  be  tied  to  it,  in  which 

'  Tin-  out, -i  i-oron:i  i-  MI  t'.iiut  that  (liDrrnnl  i>i'i>,.ii<  may  MM-  it  i|iiiti-  ilitl'rri-ntl.v.      It    i*  I'M   llir   )>iir|iosi-  of  iliDiiuJHli- 

illj-     till-.    |IITSOM.-|]||\      tliat     till-    IIM-    III'     III,'    ll|H-):l     nla~^     I-    HTIMIMIICIIlll'll. 

•  Mi-moils  iil'lhr  l.'»\:il  Aitroiiiiniiriil   Soi-irty.  Vol.   I'.',  p.   1:1.      Mr.   Slime  uilvnratril  t  hr   mi-tlioil  nl'   i-Ui-l.  Inn.;  whirl) 

.-;  lii'i  u  ,1.  -i-i  ilii-il. 


there  is  a  knot  at  a  point  just  eighteen  inches  from  the  rule.  By  holding  this  knot 
between  his  teeth,  and  keeping  the  string  stretched,  the  observer  will  be  sure  that  the 
rule  is  at  the  proper  distance  from  his  eye,  and  then  he  has  only  to  be  careful  to  hold 
it  at  right  angles  to  the  string,  lint  one  eye  should  be  used  in  making  the  measures, 
and  no  attention  need  be  paid  to  any  object  more  than  five  and  a  half  inches  from  the 
sun.  Mercury  will  be  quite  conspicuous  at  a  distance  of  seven  and  a  half  inches,  and 
Mars  will  be  seen  at  about  five  and  a  half  inches. 

6.  Very  good  observations  of  the  third  and  fourth  contacts,  the  difference  between 
which  is  the  duration  of  totality,  can  be  made  by  the  naked  eye;  the  only  instrument 
requisite  being  a  watch  having  a  seconds  hand.     The  importance  of  such  observations 
and  the  method  of  making  them  are  full}'  explained  in  Section  V. 

7.  Note  what  part  of  the  sky  is  darkest  during  totality,  the  form  of  this  dark 
part,  and  how  the  light  varies  from  it  to  the  brighter  parts. 

SECTION  II.— TO  PHOTOGRAPHERS. 

Photographs  of  the  corona  of  very  considerable  scientific  value  can  probably  be 
made  with  the  apparatus  in  ordinary  use  by  photographers.  With  this  view,  the  camera 
should  be  fitted  with  the  largest  and  longest  focused  portrait  lens  at  the  operator's  dis- 
posal, and  then,  the  diaphragm  being  removed,  it  should  be  most  carefully  focused 
upon  some  distant  object.  The  image  will  be  well  defined  only  over  a  small  space  in 
the  center  of  the  ground  glass,  but  that  is  of  no  consequence.  Two  or  three  minutes 
before  totality,  some  half-sized  plates,  prepared  with  the  most  sensitive  chemicals  ob- 
tainable, should  be  in  readiness  for  immediate  nse.  The  number  of  these  plates  should 
be  equal  to  the  number  of  half  minutes  in  the  predicted  duration  of  totality  at  the 
place  occupied  by  the  operator.  For  example,  if  the  predicted  duration  is  2m  30",  then 
five  plates  should  be  prepared.  The  camera  should  also  be  in  readiness,  directed  to 
the  sun,  and  held  there  in  any  convenient  manner.  As  the  sun  will  be  high  in  the 
heavens,  the  ordinary  camera  stand  may  riot  be  available,  but  with  a  little  ingenuity 
some  rough  contrivance  can  be  made  which  will  answer  the  purpose. 

As  many  of  the  plates  as  possible  should  be  put  in  holders  and  laid  beside  the 
camera,  so  that  they  can  be  used  without  delay.  Then,  the  focus  of  the  camera  re- 
maining as  it  was  when  adjusted  upon  the  distant  object,  as  soon  as  totality  commences, 
and  the  operator  has  assured  himself  that  the  image  of  the  corona  is  in  the  center  of 
the  ground  glass,  he  should  insert  a  plate  and  expose  it.  The  ground  glass  should 
next  be  replaced,  when,  owing'  to  the  diurnal  motion  of  the  earth,  it  will  be  seen  that 
the  image  of  the  corona  has  moved  away  from  the  center  of  the  field.  It  should  be 
piit  back,  after  which  another  plate  should  be  inserted  and  exposed;  and  this  routine 
should  be  continued  till  the  totality  is  over,  when  the  plates  can  be  developed.  With  a 
quick-acting  portrait  lens,  the  first  plate  should  be  exposed  three  seconds,  the  next  six 
seconds,  the  next  three  seconds,  and  so  on,  alternately.  With  an  ordinary  portrait 
lens  the  exposures  should  be  alternately  five  and  ten  seconds;  but  it  must  be  under- 
stood that  diaphragms  are  not  to  be  used  in  any  case.  The  development  must  be  car- 
ried far  enough  to  bring  out  all  the  details  ;  but  extreme  care  must  be  taken  not  to  push 
it  solar  as  to  clog  up  the  delicate  outlying  portions  of  the  corona.  On  account  of  the 


10 


motion  of  the  earth  while  the  plate  is  liein^  exposed,  the  intake  of  the  sun  —  or  rather 
of  the  moon,  which  covers  the  snn  —  will  he  a  little  blurred,  and  somewhat  oval  in 
shape;  and  to  prevent  disappointment,  it  must  lie  borne  in  mind  that  the  portrait  lenso 
in  ordinary  use  have  such  short  foci  that  they  can  yive  but  very  small  pictures  of  the 
moon.  The  actual  si/e  in  inches  will  be  very  nearly  as  follows,  viz:  With  a  1-4 
lens,  0.04;  with  a  1-2  lens,  0.07;  with  a  4-4  lens,  0.09  :  and  with  an  extra  4-4 
lens,  0.14.  The  diameter  of  the  extreme  outlying  portions  of  the  corona  may  be  four 
or  five  times  these  figures. 

It  is  hoped  that  photographers  making  pictures  in  this  manner  will  send  the 
original  negatives  to  the  Naval  Observatory;  but,  if  they  prefer  to  retain  them,  it  is 
earnestly  requested  that  they  will,  at  least,  send  positive  copies  on  <rlass.  1'aper 
prints  are  useless  for  scientific  purposes.  Each  plate  must  be  accompanied  by  a  state- 
ment of  the  kind  of  lens  used  in  its  production;  the  focal  distance  and  aperture  of 
that  lens;  the  chemical  process  employed  in  the  dark  room  ;  whether  the  bath  was 
neutral  or  acid;  how  lou<r  the  plate  was  exposed;  and  the  means  of  identifying  its 
top.  The  latter  point  is  of  special  importance,  and  the  upper  side  of  every  plate 
must  be  carefully  marked  when  it  is  put  into  the  holder,  because  the  character  of  the 
corona  is  such  that  without  this  precaution  it  will  be  impossible  to  say  certainly  which 
is  the  top  of  the  negative. 

SKCTIOX   III.—  Til  K    I'lIOTolIFLIooKAIMI. 

Ft.  is  known  that  very  accurate  positions  of  Mercury  and  Venus  relati\ely  to  the 
sun's  renter  can  be  obtained  from  photographs  of  the  latter  body  taken  during  transits 
of  these  planets;  and  it  inav  be  inferred  that  equally  accurate  positions  of  the  moon 
ini^lit  be  obtained  from  photographs  of  the  partial  phases  of  a  solar  eclipse.  I  n  for- 
tunately such  is  probably  not  the  case.  The  clearness  of  the  atmosphere,  the  sensi- 
tiveness of  the  chemicals  employed,  the  length  of  exposure  of  the  plate,  and  the  hour 
of  the  day,  all  affect  the  photographic  diameter  of  the  sun  to  such  an  extent  that  it  is 
practically  impossible.  to  make  a  series  of  pictures  in  which  it  shall  be  even  approxi- 
mately constant.  In  the  case  of  transits  of  .Mercury  or  Venus  this  is  of  no  consequence, 
because  it  mav  be  assumed  that  the  change  of  diameter  takes  place  uniformly  all 
around  the  circumference;  and  having  a  complete  outline  of  the  sun,  it  is  always  pos- 
sible to  find  its  center  with  accuracy.  In  a  similar  way  it  is  possible  to  find  the  center 
of  the  planet,  whose  diameter  is  also  variable,  and  thus  the  positions  of  the  two  bodies 
relatively  to  each  other  become  known.  When  the  diameter  of  a  celestial  body  is 
unknown,  at  least  a  semi-circumference  of  its  photographic  ima^e  is  nece»arv  from 
which  to  find  the  exact  position  of  its  center.  In  the  case  of  a  solar  eclipse  the  diam- 
eter- of  the  snn  and  moon  are  nearly  equal,  and  therefore  any  picture  which  shows  a 
larire  part  of  the  circumference  of  01  .....  f  these  bodies  necessarily  shows  but  a  small 
part  of  tin-  circumference  of  the  other.  For  that  reason  it  seems  impossible  to  find 
accurately  the  center  of  more  than  one  of  the  two  bodies  :  and  consequently  it  seems 
impossible  to  find  accurately  the  positions  of  the  two  bodies  relatively  to  each  other. 
On  the  other  hand,  contact  observations  afford  a  very  accurate  means  of  finding  the 
error-  of  the  lunar  tallies  :  and  as  Mich  observations  can  be  made  with  ordinary  instrn- 


11 

ments,  at  a  comparatively  trifling  cost,  it  does  not  seem  wise  to  incur  the  trouble  and 
expense  of  erecting  more  than  a  single  experimental  horizontal  photoheliograph 
specially  to  observe  an  eclipse.  At  [daces  where  such  instruments  are  already  in 
position  it  may  be  well  enough  to  use  them,  but  it  is  extremely  doubtful  if  the  pic- 
tures will  be  of  much  value. 

SECTION  IV.— THE  EQUATORIAL  CAMERA. 

Good  photographs  of  the  corona  will  be  exceedingly  valuable,  but  they  require 
a  special  form  of  apparatus  for  their  production.  That  best  adapted  to  the  purpose  is 
a  camera,  fitted  for  using  either  large  or  small  plates,  and  furnished  with  a  quick- 
acting  objective,  the  whole  being  equatorially  mounted  and  driven  by  clock-work,  so 
that  long  exposures  can  be  readily  made.  To  resist  the  strain  consequent  upon  the 
manipulation  of  the  plates,  the  mounting  and  driving  clock  must  be  of  the  most  sub- 
stantial kind;  and  to  depict  the  fainter  parts  of  the  corona  the  objective  must  be  as 
rapid  as  possible.  In  the  latter  matter  our  choice  will  be  practically  restricted  to  the 
lenses  named  in  the  following  table,  which  embraces  all  the  most  rapid  varieties  in  the 
market.  The  names  of  these  lenses,  and  the  corresponding  intensity  ratios,  are  varied 
somewhat  by  the  different  makers.  Those  given  in  the  table  have  been  taken  from 
Dalhneyer's  catalogue. 


-_        . 

<D 

•a 

d 

o 

•o 
0    a 

i? 

rt 

5 

.-        rt 

3     C 
~    ^ 

o 

Description  o!    Photographic  Objective. 

—     C 

.2    a 

"5     3 
Cfi 

£     O 

Q  "J 

v-     O 

3    ^ 

^ 

t/l 

S 

O     ^. 

o    Cr 

C.    0 

V 

pd 

C 

£-3 

a 

W 

Indus. 

/itt'/ies. 

s.           s. 

i 

1 

14 

o  3  to    I   6 

2 

Quick  Acting  Portrait       

i 

.  126 

0.7  to    3.6 

3 

Ordinary  Portrait  

1 

24 

.224 

1.3  to    6.4 

4 

Portrait  and  Group  (D)    

,', 

33 

.308 

2.9  to  14.4 

5 

Rapid  Rectilinear  

1 

33  i 

o  .  T  i  •? 

^  .  i  to  25  .  6 

If  F  is  the  equivalent  focal  distance  of  a  photographic  objective  ;  d  its  working 
aperture;  C  the  exposure  constant,  whose  value  depends  upon  the  intensity  of  the 
light  and  the  sensitiveness  of  the  chemicals  employed;  ami  £  the  time  of  exposure 

required  to  produce  a  good  negative;  then,  the  intensity  ratio  is     ,  and 


The  data  from  which  to  determine  an  approximate  value  of  ('  for  the  corona  are 
very  limited.  In  August,  1869,  M'r.  J.  A.  Whipple  obtained  a  negative  of  the  corona 
at  Shelbyville,  Ky.,  by  placing  the  sensitive  plate  at  the  principal  focus  of  an 
ordinary  astronomical  telescope.  In  this  instrument  the  value  of  i'-^tl  was  16.4,  and 
as  the  exposure  was  40  seconds  it  results  from  the  formula  above  that  (7 330.149.  In 


12 

December.  iS70,  Mr.  A.  Mrothers.  ;it  Syracuse,  Sicily,  obtained  a  very  tliin  negative 
of  the  corona,  with  a  Dalhmver  rapid  rectilinear  lens,  in  S  seconds.  For  this  objective 
/•'-=-  d=  8,  and  consequently,  ('  =  0.125.  During  the  same  eclipse,  Mr.  ().  II.  Willard, 
at  ,lere/.,  Spain,  obtained  a  negative  of  the  corona  in  90  seconds.  He  used  an  astro- 
nomical tele-cope,  precisely  as  Mr.  Whipple  did  in  August,  1869.  If  we  assume  /•'-=-</ 
to  lie  15,  as  is  usual  in  such  instruments,  then  ^'1=  0.400.  The  negative  did  not  seem 
overexposed.  In  December,  1871,  .Mr.  ,1.  IJoesinger,  at  Ootacamund,  India,  olitained 
a  negative  of  the  corona  in  3  seconds  \vith  a  Dallmeyer  No.  6  D  portrait  and  group 
lens.  In  this  case  /•'-=-  <l  •=.  6,  and,  therefore,  ('•=.  0.084.  In  August,  1869,  Dr.  Kdward 
Curtis,  at  Des  .Moines,  Iowa,  used  values  of  ('  which  were  respectively  O.Oi6  and 
O.Oii  ;  but  on  that  occasion  the  actinic  force  of  the  sun  was  diminished  from  ten  to 
tueiity  times  by  a  dense  haze,  and  therefore  these  values  were  not  equivalent  to  more 
than  O.OOI6  and  O.OO 1 1  in  an  ordinarily  clear  atmosphere.  With  the  lirst  exposure 
the  prominences  and  the  lowest  layer  of  the  chromosphere  were  well  shown.  With 
the  second  exposure  the  prominences  were  still  depicted,  but  the  chromosphere  only 
appeared  around  part  of  the  sun's  circumference.  The  corona,  properly  so  called, 
was  entirely  absent  from  both  pictures. 

From  all  these  facts  it  is  probably  safe  to  conclude  that,  with  a  clear  sky  and  a 
moderately  high  sun,  exposures  in  which  the  value  of  C"is  about  0.002  will  give  only 
the  prominences  and  the  outline  of  the  moon.  When  C  becomes  o.oS  the  corona  will 
begin  to  appear,  and  will  increase  in  extent  as  the  exposure  increases,  at  least  up  to 
the  point  where  C  becomes  040.  Beyond  that  1  am  not  aware  that  we  have  any  ex- 
perience. Accordingly,  the  shortest  exposure  specified  in  the  table  above  corresponds 
to  G'— 0.08,  and  the  longest  to  ('  —  0.40. 

Jn  choosing  a  lens  from  those  mentioned  in  the  table,  Xos.  i  and  2  may  be  at 
once  dismissed  from  consideration,  as  giving  too  small  images  of  the  sun.  Xos.  _j  and 
5  are  practically  of  the  same  focal  length,  and  as  both  of  them  are  entirely  free  from 
distortion  and  flare,  No.  4  should  be  preferred  on  account  of  its  larger  aperture. 
Thus  our  choice  is  narrowed  down  to  Nos.  3  and  4,  No.  3  having  the  greater  value  of 
/•'-=-'/,  but  neither  so  large  nor  so  Hat  a  field  as  No.  4.  If  the  lens  were  required 
solely  for  photographing  the  corona,  it  might  be  dillicult  to  choose  between  these  two, 
a-  either  of  them  will  cover  a  plate  far  larger  than  can  ever  be  required  for  that  p;,r- 
pose  ;  but  it  will  presently  appear  that  there  are  questions  in  eclipse  photography  for 
the  solution  of  which  plates  of  the  largest  si/.e  must  be  used,  and  on  that  account  it  is 
dearly  best  to  choose  No.  4. 

Having  adopted  a  lens  of  33  inches  focus,  we  have  next  to  consider  what  si/.e  of 
plates  should  be  used  with  it.  The  diameter  of  the  sun's  image  will  be  0.31  of  an 
inch,  and,  taking  .Mr.  Urothers's  picture  of  the  eclipse  of  December,  iS;o,  as  a  guide, 
the  diameter  of  the  corona  may  be  live  or  six  times  as  great,  or.  in  other  words,  about 
1.7  inches,  to  receive  which  a  plate  measuring  4  '.;  by  5'  .  inches  will  lie  ample.  Hut 
an  attempt  should  be  made  to  use  the  h-ns  for  another  purpo>e.  e\  eu  more  important 
than  photographing  the  corona.  The  magnitude  of  its  intensitv  ratio  enables  it  to 
depict  taint  objects  rapidly,  and  the  extent  of  its  angle  of  \ie\v  is  such  as  to  embrace 
a  field  of  more  than  forty  degrees.  It.  therefore,  >eeius  peculiarly  fitted  to  assist  in 


13 

the  search  for  intra-Mercurial  bodies,  provided  the  illumination  of  the  sky  does  not 
prove  too  great.  Mercury's  maximum  elongation  from  the  sun  scarcely  reaches 
twenty-nine  degrees,  and,  if  we  postulate  Bode's  so-called  law,  the  greatest  elongation 
of  the  hypothetical  intra-Mercurial  planet  or  planets  should  not  exceed  fifteen  degrees. 
Double  this  distance  is  the  space  along  the  ecliptic  which  must  be  searched;  and  to 
photograph  it  a  plate  17.7  inches  long  will  be  required.  Our  lens  will  cover  a  plate 
measuring  20  by  22  inches,  but  it  is  very  desirable  to  keep  the  apparatus  as  light  as 
possible,  and  on  that  account  it  seems  best  to  have  a  camera  twenty  inches  square, 
and  to  use  plates  measuring  i  7  by  20  inches,  which  will  suffice  to  cover  a  space  of 
thirty-three  and  a  half  degrees  along  the  ecliptic. 

Assuming  the  adoption  of  an  equatorial  camera  twenty  inches  square,  provided 
with  a  lens  whose  intensity  ratio  is  one-sixth  and  whose  focal  distance  is  about  thirty- 
three  inches,  it  yet  remains  to  consider  how  this  apparatus  should  be  managed  during 
a  totality  lasting  only  three  minutes.  As  the  illumination  of  different  parts  of  the 
corona  varies  greatly,  there  can  evidently  be  no  certainty  of  getting  all  the  details  of 
the  phenomenon  unless  a  series  of  plates  are  taken,  in  which  the  exposures  vary  from 
the  shortest  possible  up  to  the  point  where  it  is  certain  that  an  increase  of  time  does 
not  improve  the  picture.  On.  this  account  it  will  be  desirable  to  take  as  many  as  six 
plates,  the  exposures  being,  respectively, 

3s,  5s.  ios,  2OS,  40",  and  6os.* 

The  first  four  of  these  plates  will  receive  such  short  exposures  that  it  is  unlikely  they 
will  show  anything  but  the  corona,  and  therefore  their  size  should  be  4^  by  y/2  inches. 
With  the  last  two  plates  the  case  is  different.  Their  size  should  be  17  by  20  inches, 
because  their  longer  exposures  will  probably  suffice  to  bring  out  upon  them  any  bright 
points  which  may  exist  within  their  field.  A  lens  such  as  is  here  under  consideration 
should  depict  an  eighth-magnitude  star. in  about  one  minute,  but  of  course  the  inten- 
sity of  the  sky-illumination  during  totality  will  determine  the  limit  of  brightness  at 
which  faint  luminous  points  will  cease  to  impress  themselves  upon  the  negatives,  and 
what  this  limit  may  be  it  is  impossible  to  predict,  The  necessity  for  at  least  two 
large  plates  is  evident  when  it  is  remembered  that  the  image  of  a  small  bright  point 
could  not  be  distinguished  from  an  accidental  blemish  in  the  film,  and  it  wquld  only 
l)e  by  finding  it  upon  both  plates  that  its  true  character  could  be  unmistakably  recog- 
nized. It  is  exceedingly  desirable  to  determine  accurately  the  maximum  exposure 
that  the  corona  will  bear  with  advantage,  and  it  is  hoped  that  on  at  least  one  of  the 
large  plates  it  will  [trove  to  be  over-exposed. 

For  the  management  of  the  equatorial  camera  two  persons  will  be  required,  desig- 
nated respectively  as  No.  i  and  No.  2,  and  the  following  routine  will  be  found  conven- 
ient. The  operations  in  the  dark  room  must  be  so  managed  that  the  six  sensitive 
plates  shall  be  placed  beside  the  camera,  in  readiness  for  instant  use,  a  minute  or  two 

'  It  is  questionable  whether  or  not  all  these  exposures  can  be  made  in  tbiee,  minutes,  but  a  few  preliminary  trials 
will  settle  that  point.  Experiments  should  also  be  made  upon  the  stars  in  the  evening  twilight  to  ascertain  if  an  ex- 
posure so  long  as  sixty  seconds  is  really  required.  In  any  event,  it  will  probably  be  best  to  reverse  the  order  given 
above,  and  to  work  oft'  first  those  plates  requiring  most  time,  thus  reducing  to  a  minimum  the  damage  which  would 
accrue  from  the  sun  bursting  out  while  an  exposure  was  in  progress. 


14 

tin-  predicted  time  of  Mvoiid  contact.  Six  sand-glasses  must  also  lie  pro- 
vided, each  adjusted  to  run  for  the  number  of  seconds  that  the  plate  to  which  it 
belongs  is  to  lie  exposed.  Then,  the  cap  being  on  the  lens,  the  first  plate  must  he 
placed  in  the  camera,  and  the  slide  must  lie  withdrawn  from  the  holder:  after  which 
operator  No.  i  will  take  position  with  his  hand  upon  the  cap  covering  the  lens,  and 
No.  2  will  seize  the  proper  sand-glass  and  hold  it  in  such  a  way  that  the  sand  shall 
all  he  in  the  bottom  of  the  glass.  At  the  instant  totality  begins,  No.  I  will  say, 
"  Ueady?"  No.  2  will  answer,  "Ready."  No.  I  will  begin  the  exposure  of  the 
plate  bv  careful! v  removing  the  cap  from  the  lens,  exclaiming  as  he  does  so,  "Turn;" 
when  No.  2  will  instantly  turn  the  glass.  Moth  operators  will  stand  motionless 
while  the  sand  is  running,  No.  2  watching  carefully  for  the  escape  of  the  last 
grain,  when  he  will  exclaim,  "Out,"  and  No.  i  will  instantly  terminate  the  expos- 
ure by  capping  the  lens.  Without  a  moment's  delay  No.  i  will  push  the  coyer  into 
the  plate-holder,  remove  the  exposed  plate,  place  the  next  holder,  containing  a  fresh 
plate,  in  the  camera,  and  draw  the  slide;  being  assisted  in  all  this  by  No.  2,  if  neces- 
sary. Then  No.  i  will  again  take  position-  with  his  hand  upon  the  cap  of  the  lens; 
No.  2  will  seize  the  second  sand-glass;  and  this  plate  will  be  exposed  in  the  same 
manner  as  the  preceding.  This  routine  will  be  continued  till  all  the  plates  have  been 
exposed,  and  after  totality  is  over  they  n'ill  be  developed  in  the  usual  way. 

With  regard  to  the  plate-holders,  it  may  be  remarked  that  the  last  two  plates 
must  necessarily  he  in  holders  as  large  as  the  camera,  but  the  first  four  plates  can  he 
more  conveniently  handled  if  they  are  in  half-sized  holders.  To  render  such  an 
arrangement  possible  a  reducing  frame  must  be  provided,  which,  upon  being  applied 
to  the  camera,  will  reduce  its  opening  to  that  proper  for  receiving  the  small  holders. 

As  to  the  photographic  process  to  be  employed,  if  only  small  pictures  are  desired, 
dry  plates,  prepared  with  washed  emulsion,  are  recommended;  but  if  any  large 
plates  are  to  be  taken  the  wet  collodion  process  will  be  preferable.  In  either  case  it 
is  specially  important  to  use  well  bromized  collodion,  because  the  bulk  of  the  coronal 
light  has  probably  a  wave  length  of  5315.9  (the  14/4  line),  and  to  such  light  silver 
bromide  is  far  more  sensitive  than  either  silver  chloride  or  silver  iodide.  For  wet 
plates  there  is  reason  to  think  that  fineness  of  deposit  and  richness  in  details  can  be 
best  secured  by  a  free  use  of  sugar  in  the  iron  developer. 

To  facilitate  the  study  of  the  red  prominences,  it  has  been  proposed  to  photograph 
them  on  a  scale  of  ten  seconds  of  arc  to  a  millimeter.  The  optical  apparatus  for  the 
production  of  such  pictures  must  have  an  equivalent  focal  distance  of  2062.7  centi- 
meters, or  8  i  2.  i  inches,  and  if  we  take  ('  equal  to  O.OO2,  which  is  probably  very  near 


15 


the    truth,*  the  values  of  t   for  lenses  of  various  apertures  can  be  found  from  the 
formula  already  given,  and  will  be  as  follows  : 


V* 

CJ 

l_i 

O    o 

'•+      *r-t 

o 

2  •- 

3    H 

c    c* 

3    o 

F 

(                 J 

in     v" 
0     3 

0     0 
.—     o 

'-*   •*—  . 

r/ 

V  (?  / 

0.    f 

<u  ^ 

«     ° 

o  •* 

<  ° 

bi 

s 

Indies. 

J". 

., 

6 

135-3 

18306 

36.6 

20.1 

8 

IOI.5 

10302 

20.  6 

H-3 

10 

Si.  2 

f>593 

13-2 

7-2 

12 

67.7 

45S3 

9.2 

5-0 

15 

54-1 

2927 

5-8 

3-2 

2O 

40.6 

1648 

3-3 

1.8 

26 

31-2 

973 

1.9 

I.O 

The  last  column  of  the  table  gives  the  moon's  motion  during  an  exposure  of  the 
length  specified  in  the  preceding  column.  A  prominence  one  minute  high  is  a  large 
one,  and  yet  it  could  scarcely  be  photographed  with  a  six-inch  objective,  because  twenty 
seconds  of  its  height  would  be  covered  by  the  advancing  moon  before  the  exposure 
was  over.  In  short,  it  does  not  seem  possible  to  photograph  prominences  during 
eclipses  on  the  scale  here  contemplated  with  an  aperture  much  less  than  ten  inches, 
and  twelve  woiild  be  far  better.  Under  these  circumstances  the  practicability  of  the 
scheme  is  questionable. 

SECTION  V.— TELESCOPIC  OBSERVATIONS. 

Ti'lexcojH's. — A  proper  telescope  for  observing  the  eclipse  is  very  desirable,  but 
any  telescope  at  all  is  better  than  none ;  and  owners  of  small  spy-glasses,  or  opera- 
glasses,  should  bear  in  mind  that  their  instruments  are  capable  of  doing  good  service. 
( )pera-glasses  can  be  held  in  the  hand  with  sufficient  steadiness,  but  spy-glasses,  how- 
ever small,  will  require  a  stand  to  produce  the  best  effect.  This  need  not  be  an 
elaborate  affair,  but  may  be  made  of  wood  in  the  simplest  manner,  provided  only  that 
it  is  capable  of  holding  the  telescope  steadily  pointed  in  any  required  direction.  Some 
artificial  support  is  absolutely  necessary,  and,  in  default  of  anything  better,  the  tele- 
scope might  even  be  lashed  to  a  round  post. 

Sh(i</<'  (ildSM'.i. —  Fit  anticipation  of  the  late  transit  of  Mercury,  several  letters  Avere 
received  at  the  Naval  Observatory  inquiring  about  shade  glasses,  and  on  that  account 
it  seems  desirable  to  notice  the  subject  here.  The  best  form  of  shade  is  undoubtedly 
.a  long  wedge  of  colored  glass,  achromatized  by  a  similar  wedge  of  white  glass  firmly 
cemented  to  it.  The  most  usual,  and  by  tar  the  most  objectionable,  color  for  a  shade 
is  red.  Either  neutral  tint  or  green  is  excellent,  but  it  is  difficult  to  obtain  the  latter 
sufficiently  dark.  Deep  blue  has  been  advocated  by  some  observers,  and  dec})  }'ellow 
was  much  used  in  Germany,  but  is  now  apparently  losing  favor.  Shades  consisting 
of  a  single  piece  of  thick  glass  are  very  apt  to  split,  and  thus  endanger  the  observer's 

*Tho  exposures  given  by  Mr.  Do  la  Rue  to  his  pictures  of  tho  eclipse  of  July  iS,  1860,  indicate  a  value  of  C  some- 
where between  0.0007  all(l  0.0012  ;  while  Dr.  Curtis's  exposures  on  the  eclipse  of  August  7,  1869,  give  ('  =0,0016. 


16 

eye,  it'  tin-  rays  of  ;i  bright  sun  arc  concentrated  upon  them  Ity  an  objective  of  lame 
aperture.  To  avoid  this,  such  objectives  must  have  their  aperture  reduced  by  a  cap, 
t\\n  inclio  hciujr  tho  proper  si/.c  for  a  telescope  of  thirty  inches  focus,  and  three  inches 
for  one  of  live  or  six  feet  focus.  My  making  the  shade  of  three  thicknesses  of  "-lass, 
the  piece  next  the  eye  heiny  thickest  and  darkest  in  color,  the  other  pieces  bciii^- 
successively  thinner  and  lighter  in  color,  and  all  hcinir  fitted  loosely  into  their  cell  so 
that  they  can  expand  freely,  it  will  be  possible  to  use  with  safety  an  aperture  of'five 
inches  upon  a  telescope  of  six  feet  focus.  When  no  proper  shade  is  at  hand,  an 
excellent  substitute  can  be  made  by  smoking  a  slip  of  thin  Hat  ii'lass,  about  an  inch 
wide  and  four  inches  Ion**1,  in  such  a  way  that  its  tint  may  increase  uniformly,  from 
very  li-i'ht  at  one  end  to  a  d'-pth  at  the  other  end  stillicient  for  viewing  the  brightest 
sun  without  inconvenience.  The  smoked  surface  should  be  protected  b\-  a  covering 
jjlass,  of  the  same  si/.e  as  the  original  slip,  and  separated  from  it  by  bits  of  thin  card 
inserted  at  the  ends,  the  whole  heiii^-  bound  together  by  a  narrow  strip  of  paper  pasted 
around  the  ed^es.  For  observing  the  second  contact  of  the  eclipse,  a  neutral  tint,  or 
li-reeii.  spectacle  <,rlass  will  be  found  convenient, 

Of  course  it  will  be  understood  that  the  necessity  for  caps  to  diminish  the  aper- 
ture is  confined  to  common  telescopes.  Instruments  intended  for  observing  the  sun 
are  always  provided  with  arrangements  for  ^ettin»-  rid  of  the  excessive  li^ht  and  heat 
without  diminishing  the  aperture,  and  often  without  usin^  shades 

lli'dthif/  <>f  TelescojH'. — When  observing  a  brio-lit  sun  the  eye  end  of  the  telescope 
soon  becomes  quite  hot,  and  thus  air  currents  an*  set  up  in  the  tube  which  interfere 
seriously  with  the  definition.  To  remedy  this  the  tube  must  occasionally  be  cooled, 
either  by  shading  the  objective  or  by  turning  the  telescope  away  from  fly  sun  As 
it  is  important  to  have  the  best  possible  definition  while  observing  the  contacts  and 
totality  of  the  eclipse,  special  care  must  be  taken  that  the  temperature  of  the  lube  at 
these  times  is  the  same  as  that  of  the  surrounding  air. 

Tlic  (  oittm-lt;. — The  first  touch  of  the  moon  upon  the  sun's  limb  is  technically 
called  the  first  contact,  the  complete  extinction  of  the  last  rav  of  sunlight,  is  simulta- 
neous with  the  second  contact,  the  reappearance  of  the  minutest  possible  portion  of 
the  sun  marks  the  third  contact,  and  tho  disappearance  of  all  trace  of  the  moon  from 
the  sun's  disk  constitutes  the  fourth  or  last  contact.  The  first  and  fourth  contacts  are 
sometimes  called  the  external,  and  the  second  and  third  the  internal,  contacts.  The 
observation  of  these  contacts  consists  in  noting  the  exact  instants  of  their  occurrence, 
and  for  that  purpose  a  time-piece  with  a  seconds  hand  and  a  telescope  are  usually 
employed,  hut  if  the  latter  instrument  is  not  obtainable,  the  internal  contacts  can  be 
well  observed  with  the  naked  eye. 

There  is  generally  some  uncertainty  about  the  first  contact,  because  the  only  evi- 
dence of  its  occurrence  is  a  slight  indentation  in  the  sun's  limb,  which,  unless  the  ob- 
server knows  precisely  where  to  look  for  it,  naturally  attains  some  si/e  before  it  is  seen, 
ami  thus  the  contact  is  noted  too  late.  If  the  telescope  has  a  position-micrometer,  its 
wire  may  be  set  at  the  computed  point  of  first  contact,  and  then  there  \\ill  be  no  dilli- 
culty ;  but  in  the  absence  of  such  an  arrangement,  the  only  way  is  to  use  a  power  of  not 
more  than  sixty  diameters,  and  to  watch  carefully  a  portion  of  the  sun's  circumference 


17 

large  enough  to  include  with  certainty  the  point  in  question.  The  shade-glass  enl^- 
ployed  should  be  sufficiently  dark  to  destroy  all  glare,  and  to  permit  of  the  sun's  im- 
age being  viewed  without  effort. 

The  internal  contacts  are  phenomena  of  such  a  definite  character  that  the  instants 
of  their  occurrence  "can  be  noted  within  a  small  fraction  of  a  second.  For  a  minute 
or  two  before  the  predicted  time  of  second  contact  the  sun's  thin,  and  now  fast  wan- 
ing, crescent  should  be  carefully  watched  through  a  neutral  tint  or  green  shade  glass 
as  light  as  the  eye  can  conveniently  bear.  Presently  the  crescent  will  become  a  mere 
thread  of  light,  which  will  rapidly  shorten,  and  suddenly  disappear.  This  is  the  second 
contact,  and  there  can  be  no  mistake  about  the  exact  instant  of  its  occurrence  because 
the  ruddy  glow  of  the  chromosphere,  which  remains  at  the  point  last  covered  by  the 
moon,  is  totally  different  from  the  dazzling  white  light  of  the  photosphere  which  has 
just  disappeared.  Through  a  red  shade  glass  the  contrast  between  these  two  kinds  of 
light  might  be  partly  lost,  but  through  a  neutral  tint  or  green  glass  it  will  be  suffi- 
ciently apparent.  Finally,  there  is  yet  another  indication  of  the  second  contact,  and 
that  is  the  seemingly  miraculous  appearance  of  the  complete  outline  of  the  moon, 
round  and  black,  reposing  upon  the  wondrous  radiance  of  the  corona. 

The  approach  of  the  third  contact  will  be  heralded  by  the  rapid  brightening  of 
the  chromosphere  at  the  point  of  the  moon's  limb  where  the  sun  is  about  to  reappear, 
and  two  or  three  seconds  later  a  sudden  burst  of  light  will  announce  the  contact  itself 
and  with  it  the  termination  of  totality.  This  contact  may  be  observed  without  any 
shade  glass. 

The  observation  of  the  fourth  contact  is  a  very  simple  matter.  The  segment 
cut  out  of  the  sun  by  the  retreating  limb  of  the  moon  is  carefully  watched  as  it  be- 
comes less  and  less,  and  the  instant  of  its  final  disappearance  is  noted  as  the  fourth 
contact.  The  shade  glass  used  must  be  so  dark  as  to  destroy  all  disagreeable  glare. 

To  observe  the  contacts,  a  station  must  be  chosen  where  there  is  an  uninterrupted 
view  of  the  sun,  and  the  party  must  consist  of  only  two  persons,  one  of  whom  must 
have  a  time-piece  and  writing  materials  and  the  other  must  have  a  telescope.  With 
regard  to  the  latter  instrument,  it  may  be  remarked  that  if  it  is  a  common  one — a  spy- 
glass, for  instance — there  will  probably  be  no  choice  as  to  its  magnifying  power;  but 
if  it  belongs  to  the  better  class,  and  has  a  battery  of  eye-pieces,  the  power  chosen 
should  be  equal  to  fifteen  or  twenty  times  the  diameter  of  the  objective  in  inches.  To 
secure  the  best  results,  the  party  must  be  entirely  alone;  and  above  all,  it  must  not 
Ite  within  hearing  of  other  observing  parties.  A  minute  or  so  before  the  expected 
time  of  contact  the  person  with  the  time-piece  will  fix  his  eyes  upon  its  seconds  hand 
and  begin  silenth*  to  count  the  seconds,  while  the  observer  at  the  telescope  will  care- 
fully watch  the  sun.  Upon  the  occurrence  of  contact  the  observer  will  cry  "Mark," 
and  the  person  at  the  time-piece  will  immediately  write  down,  first,  the  second;  then, 
the  minute ;  and,  finally,  the  hour.  Before  beginning  to  observe,  care  must  be  taken 
to  see  that  when  the  seconds  hand  points  to  60  the  minute  hand  is  exactly  over  a 
minute  mark;  and  in  observing,  if  the  seconds  are  near  sixty,  special  care  will  be  re- 
quired to  avoid  recording  a  wrong  minute.  For  example,  when  the  time  is  4"'  58"  the 
minute  hand  will  be  almost  exactly  over  5"' ;  and  special  care  must  be  taken  not  to 
3 


18 

record  5™  58*  instr;id  of  4™  58'.  Practice  in  calling  and  noting  time  previous  to  the 
eclipse  will  greatly  diminish  the  risk  of  such  mistakes.  A  little  before  totality  a  lan- 
tern should  be  lighted,  because  without  it  there  may  be  difliculty  in  seeing  the  hands 
of  the  time-piece  at  the  approach  of  third  contact. 

Observations  of  the  internal  contacts  can  be  utilized  without  a  knowledge  of  the 
error  of  the  time-piece ;  but  with  the  external  contacts  the  case  is  dill'erent.  They 
will  be  of  no  value  unless  the  error  of  the  time-piece  is  accurately  known,  either  on 
local  or  on  Washington  time.  To  meet  tliis  difliculty,  it  is  hoped  that  arrangements 
may  be  made  by  which  the  Western  Union  Telegraph  Company  will  furnish,  free  of 
cost,  a  signal  from  the  Naval  Observatory,  indicating  the  instant,  of  Washington  mean 
noon,  to  all  proper  persons  who  may  apply  for  it  at  any  of  their  ollices  on  July  27, 
29,  and  30.  The  intending  observer  can  best  utili/e  these  signals  by  making  sure 
that  the  minute  and  seconds  hands  of  his  watch  a»rec  with  each  other,  as  explained 
above,  and  then  carrying  the  watch  to  the  telegraph  otlice  on  each  of  the  days  men- 
tioned and  noting  the  hour,  minute,  and  second  indicated  by  it  when  the  signal  is  re- 
ceived. No  matter  how  the  watch  may  run,  it  must  not.  be  meddled  with  in  any  wav, 
either  by  moving  its  hands  or  touehing  its  regulator,  between  July  26  and  31.  With- 
out this  precaution,  neither  its  rate,  nor  the  degree  of  dependence  to  be  placed  upon 
it,  can  be  determined. 

The  final  record  of  a  set  of  observations  of  contacts  should  contain  the  following 
information : 

1.  The  name  of  the  station;  including  town,  county,  and  State. 

2.  The  date. 

3.  A  description  of  the  location  of  the  station  with  reference  to  the  nearest  promi- 
nent landmarks;  as,  for  example,  its  bearing  and  distance  from  the  nearest  court-house, 
railroad  station,  or  church  ;   or  its  position  with  respect  to  the  lines  of  the  I'nited  States 
land   surveys;  or  the  bearing  and  distance  of  a   prominent  mountain  peak;  or  the 
bearings  of  three  or  more  such  mountain  peaks;  or,  in  the  case  of  a  citv,  the  street 
and  number,  together  with  the  names  of  the  nearest  cross  streets  on  each  side. 

.   4.  A  description  of  the  time-piece,  and  a  statement  of  the  aperture,  focal  length, 
and  magnifying  power  of  the  telescope  emploved. 

5.  The  times  indicated  by  the  watch  at  the  reception  of  the  Washington  noon 
signal  on  July  27,  29,  and  30. 

6.  The  time  of  each  of  the  contacts;  just  as  read  from   the  face  of  the  watch, 
without  the  application  of  any  correction. 

7.  An  estimate  of  the  uncertainty  to  which  these  times  are  liable. 
S.  Any  remarks  which  may  be  thought  necessarv. 

9.  The  signatures  of  the  two  observers,  thus:    A.  1!.,  observer  with  telescope;    (' 
1  > ..  time  observer. 

It  is  particularly  requested  that  the  original  pencil  memoranda  of  the  times  of 
contact  lie  s,.nf  to  the  Naval  Observatory,  inclosed  in  the  above-described  report. 

If  several  parties  observe  the  eclipse  in  the  .same  neighborhood,  it  is  quite  likely 
that  their  recorded  times  may  differ  a  little.  Such  differences  occur  even  in  the  work 
of  the  most  practiced  astronomers;  and  thev  must  be  allowed  f<>  remain.  It  is  never 


19 

permissible  to  alter  the  record.     A  mere  suspicion  that  sucli  a  thing  has  been  done 
will  insure  the  entire  rejection  of  the  observation. 

Limits  of  the  Shadow  Path. — There  is  yet  so  much  uncertainty  in  the  best  solar 
and  lunar  tables  that  in  the  eclipse  of  July  next  the  British  Ephemeris  places  the  path 
of  the  moon's  shadow,  while  crossing  Colorado,  about  three  and  a  half  miles  south- 
west of  the  position  assigned  by  the  American  Ephemeris.  It  is  therefore  important 
to  determine  accurately  the  true  position  of  the  path,  and  this  may  be  readily  accom- 
plished by  observing  the  duration  of  totality  at  points  situated  from  one  to  ten  miles 
within  the  shadow.  As  the  duration  of  totality  is  the  interval  between  the  second  and 
third  contacts,  it  is  determined  by  observing  these  contacts  in  the  manner  already 
described;  except  that  for  this  purpose  a  knowledge  of  the  error  of  the  watch  is  unnec- 
essary, and  the  observations  may  be  made  with  the  naked  eye  if  a  telescope  is  not 
available.  At  places  very  near  the  edge  of  the  shadow  the  totality  may  be  so  brief 
that  the  time  observer  would  risk  losing  the  third  contact  if  he  stopped  to  record  the 
second.  At  such  places  it  will  be  necessary  to  have  a  third  person  to  make  the  record, 
and  then  both  contacts  can  be  readily  observed,  even  if  they  follow  each  other  at  an 
interval  as  short  as  a  single  second. 

.  Sketches  of  the  Corona. — In  making  a  general  sketch  of  the  corona  the  magnifying 
power  employed  should  not  exceed  from  twenty  to  forty  diameters,  and  some  me- 
chanical means  should  be  adopted  to  insure  accuracy  of  outline.  Owing  to  the  faint- 
ness  of  the  corona,  it  is  doubtful  if  the  camera  lucida  can  be  successfully  used;  and 
the  most  promising  plan  seems  to  be  a  system  of  squares,  ruled  on  glass,  in  the  focus 
of  the  eye  piece  of  the  telescope,  and  a  corresponding  system  drawn  upon  the  paper 
on  which  the  sketch  is  .to  be  made.  It  is  known  that  spider  lines  in  a  micrometer 
sometimes  cease  to  be  visible  under  the  feeble  illumination  of  the  corona,  and  to  avoid 
the  possibility  of  this  happening  with  the  squares,  the  "lines  defining  them  should  be 
rather  coarsely  cut.  More  than  once  attempts  have  been  made  to  use  a  telescope  after 
the  manner  of  a  camera  obscura,  by  drawing  out  the  eye  piece  a  little,  and  thus 
causing  it  to  throw  an  enlarged  image  of  the  corona  upon  a  piece  of  finely  ground 
glass,  or  semi-transparent  paper,  placed  a  few  inches  beyond ;  the  whole  being  so 
arranged  that  the  image  could  be  seen  through,  and  traced  upon,  the  glass  or  paper. 
Owing  to  the  faintness  of  the  image,  such  attempts  have  not  hitherto  been  very  suc- 
cessful, but  perhaps  they  might  do  better  if  for  the  plate  of  ground  glass  a  thin  sheet 
of  gelatine,  or  mica,  were  substituted,  and  the  drawing  were  traced  upon  it  with  a 
needle  point.  The  proper  mode  of  executing  such  sketches  has  been  already  described 
in  Section  I. 

Miit/i/c  Examination  of  the  Corona. — With  as  high  a  magnifying  power  as  the 
telescope  will  readily  bear,  examine  the  structure  of  the  corona;  noting  particularly 
whether  it  is  homogeneous,  nucleated  like  a  star  cluster,  or  filamentous.  If  any  nuclei 
are  seen,  describe  their  shape,  size,  color,  and  mode  of  distribution.  If  filaments  are 
discerned,  state  their  length,  diameter,  color,  and  mode  of  distribution ;  whether  they 
are  straight,  curved,  or  contorted ;  in  what  direction  they  lie  relatively  to  the  sun  ;  and, 
if  curved,  specify  how  the  curvature  faces.  Be  sure  to  observe  how  the  structure 
varies  in  passing  from  the  sun  to  the  outermost  visible  limit  of  the  corona. 


20 

Since  the  spectroscope  furnishes  an  efficient  means  of  studying  tin-  red  promi- 
nences at  any  time,  it  will  be  very  umlesiraltle  to  waste  a  single  one  of  the  precious 
nioineiits  of  totality  in  examining  them. 

Intr<i-MiT<:nrinl  llotlic*. — To  facilitate  the  work  of  such  astronomers  as  may  desire 
to  .-earch  for  intra-Mercurial  planets  with  considerable  telescopic  power,  ;i  chart  will 
lie  found  at  the  end  of  these  instructions  showing  every  star  so  large  as  the  seventh 
magnitude  in  that  portion  of  the  heavens  which  will  be  occupied  by  the  sun  on  the 
sgth  of  July  next.  The  black  circle  in  K.  A.  X1'  36'",  Dec.  +  18°  39'  indicates  the 
position  of  the  sun.  Mercury,  Kegulus,  and  Mars  will  be  pretty  close  together,  and 
probably  quite  conspicuous  during  totality,  but  they  are  so  far  to  the  eastward  that 
only  the  last-named  conies  within  the  limits  of  the  chart.  Venus  may  also  be  seen, 
but  she  will  be  low  in  the  western  sky.  While  looking-for  planets,  the  possibility  of 
discovering  a  small  comet,  or  a  meteor  stream,  should  be  borne  in  mind. 

I'dftia!  J'/KISCS. — The  corona  forms  a  luminous  background  upon  which  the  moon's 
limb  is  sometimes  seen  projecting  beyond  the  sun;  and  a  little  before  totality  it  is 
even  possible  that  the  complete  outline  of  the  moon  may  become  visible.  Look  for 
these  phenomena,  and  note  the  time  of  their  occurrence.  It  is  difficult  to  assign  any 
reason  for  the  existence  of  rays,  or  brushes,  of  light  at  the  cusps  of  the  sun,  lnit.it  is 
saiil  thev  have  been  seen.  If  any  such  appearances  present  themselves,  they  should  he 
earefullv  scrutinized  to  ascertain  if  they  change  either  their  position  or  intensity  ;  and 
the  interior  of  the  telescope  should  be  examined  to  make  sure  that  they  do  not  ori^i 
uate  in  reflections,  either  from  the  tube,  or  from  the  lenses 

SKCTION  VI.— SPECTROSCOPIC  OHSKUVATloNS. 

This  section  has  been  largely  compiled  from  the  writings  of  Young  and  l.ockyer, 
some  of  the  paragraph*  being  almost  in  the  exact  language  of  these  authors.* 

Among  the  numerous-  arrangements  of  spectroscopic  apparatus  devised  by  scientific 
men,  experience  has  shown  that  those  mentioned  below  are  best  adapted  to  the  obser- 
vation of  eclipses,  viz  : 

Aini///:iin/  IiiNtritiiH'iit,  A. — A  spectroscope  having  a  long  slit  and  a  long  collimator, 
mounted  at  right  angles  to  the  optical  axis  of  a  reflecting  telescope  of  large  aperture 
and  short  focus;  the  slit  of  the  spectroscope  lying  in  the  focus  of  the  mirror  as  usual 
and  the  telescope  being  provided  with  a  large  finder.  In  this  combination  the  ima^e 
of  the  corona  is  very  bright,  and  small  enough  in  proportion  to  the  length  of  the  slit 
to  allow  of  the  simultaneous  visibility  of  the  spectrum  on  both  sides  of  the  dark  moon  ; 
while  the  long  collimator  permits  the  use  of  a  wide  slit,  and  thus  insures  the  admission 
of  the  maximum  of  light. 

Iiitri/fiiliiii/  Inxtnitiiritf,  11. — A  spectroscope,  having  a  long  collimator,  mounted  so 
that  the  light  falling  upon  its  slit  is  received  from  the  eve-piece  of  a  telescope  of 
large  aperture  and  short  focus,  previously  adjusted  for  distinct  vision  on  a  distant 

"  See  tin-  following:   Keport  on  Observations  of  the  Total  Solar  Kelipse  of  December  22,  1870.     Jiy  Pi  of.  (.'.  A.  YOIIII-J 
U.  8.  Coast  Silrvpy  Re|»ort  for  1X70,  p.  141.     Inst ructions  for  Observers  at  tin-   Kn<;lisli  (iovei  mnent  l-'.clipse  Kxpedilion 
1-71.     Nature.  1X71,  vol.  4,  p.  517.     Knglixli  (ioveriiincnt  Kelipue  r.\pedilion,  1X75       Instniriions  to  Observers.      Nairn.-, 
•875,  vol.  II,  p.  351.      Report  on  tin'  Total  Solar  Keli|ise  of  April  6,  1X75.      l!y  .1.  \.  l.oekyer  and   Arllnir  Schuster.      I'liil. 
Tiauit.     Tbu  Coming  Total  Solar  Kelipse.     liy  .1.  X.  l.oekyei.     Nature.  |S;X,  vol.  17.  pp.  481  and  501. 


21 

object.  The  area  of  the  heavens  from  which  this  telescope  collects  its  light  should  be 
three  or  four  degrees  in  diameter,  which  will  be  the  case  if  it  magnifies  about  fifteen 
times.  To  insure  accurate  pointing  it  must  be  provided  with  a  finder. 

Analyzing  Instrument,  0. — An  automatic  spectroscope  of  large  dispersive  power ; 
attached  to  an  equatorial  telescope  in  the  usual  manner. 

Aini/i/.i/i/(i  Instrument,  D. — A  spectroscope  of  moderate  or  small  dispersive  power; 
attached  to  ci  telescope  as  above  described. 

[utcyratiny  Instrument,  E. — A  spectroscope,  either  mounted  upon  a  stand  or  held 
in  the  hand,  receiving  the  light  of  the  corona  directly  upon  its  slit  without  the  inter- 
vention of  any  optical  apparatus  whatever. 

Iitti'f/i-dfh/i/  litxtnimeiit,  F. — A  meteor  spectroscope,  having  neither  slit  nor  colli- 
mator,  but  receiving  the  light  directly  upon  its  prism. 

The  telescopes  to  which  the  above-described  spectroscopes  are  attached  should 
nil  be  equatorially  mounted ;  and  they  must  either  be  driven  by  clock-work,  or 'an 
assistant  must  be  employed  to  point  th'em.  When  a  spectroscope  slit  is  covered  from 
view  the  only  available  mode  of  bringing  an  object  upon  it  is  by  means  of  the  finder; 
but  if  it  is  exposed,  as  it  should  always  be,  the  image  of  the  object  upon  the  slit-plate 
will  be  the  best  guide.  For  observing  the  corona  the  width  of  the  slit  may  be  such 
jis  to  show  but  two  of  the  three  h  lines  when  the  spectroscope  is  directed  to  a  faint 
cloud.  The  instruments  best  adapted  for  making  the  observations  suggested  below 
have  in  every  case  been  indicated  by  the  use  of  the  reference  letters  employed  in  the 
descriptions  of  apparatus  just  given. 

/irttxltcs  oflif/ht  af  ciixjtH. — Some  observers  report  having  seen  brushes  of  red  light 
at.  the  solar  cusps  during  the  partial  phases  of  eclipses.  It  is  difficult  to  understand 
how  such  phenomena  can  occur,  but  if  they  present  themselves  their  spectra  should 
be  carefully  examined.  Instrument,  C  or  D. 

Yoiuiifs  llei'ervhif/  Layer.— -Close  to  the  sun's  limb  look  for  the  layer  in  which  the 
Fraunhofer  lines  originate.  Just  before  totality  it  should  give  a  nearly  continuous 
spectrum,  and  just  at  the  instant  of  totality  it  should  show  reversed  all  the  dark  lines 
of  the  spectrum,  except  those  of  terrestrial  origin.  The  dispersive  power  of  the  spec- 
troscope employed  cannot  be  too  high.  The  image  of  the  sun  should  not  be  less  than 
one  inch  in  diameter,  and  since  the  thickness  of  the  layer  in  question  does  not  exceed 
2",  it  is  evident  that,  extreme  care  must  be  exercised  in  adjusting  the  focus,  in  making 
the  slit  very  narrow,  and  in  placing  it  precisely  in  the  plane  of,  and  rigorously  tan- 
gent to,  the  solar  image.  Instrument,  C  or  D. 

An  attempt  should  be  made  to  measure"  the  thickness  of  the  reversing  layer  by 
noting  how  long  the  lines  continue  bright.     But  for  this  purpose  a  chronograph  capa-, 
ble  of  indicating  the  one  hundredth  part  of  a  second  will  be  necessary. 

The  Prominence  b^n'cfnnn. — Just  before  and  just  after  totality  the  atmospheric 
glare  is  greatly  reduced,  and  advantage  should  be  taken  of  this  circumstance  to  look 
carefully  for  new  lines.  The  search  should  be  directed  particularly  to  that  part  of  the 
spectrum  lying  above  F,  as  it  deserves,  and  will  probably  repay,  careful  study,  and  is 
very  difficult  to  deal  with  in  the  absence  of  an  eclipse.  Instrument,  C. 


22 

During  totality,  attend  to  the  following  points: 

In  passing  outward  from  the  sun,  are  the  li  linos  replaced  by  a  diffuse  band?. 
Does  the  /•'  line  extend  higher  than  the  other  hydrogen  lines  .'  Instruments,  J,  (',  l>. 

It  !>  known  that  the  prominences  consist  mainly  of  hydrogen;  but  do  they  ever 
contain  large  quantities  of  calcium  .'  In  other  words,  are  the  //  lines  sometimes  con- 
spicuous in  them  .'  Instruments,  .1,  (',  I). 

Probably  the  //  line  is  produced  only  by  very  hot  hydrogen.  As  giving  a  clew  to 
the  temperature  of  the  prominences,  look  for  it  in  their  spectrum,  and  note  how  high 
above  the  photosphere  it  exists.  Instruments,  A,  C,  />. 

Look  for  new  lines  in  the  ultra- violet  part  of  the  spectrum.  Instrument,  Cor  />, 
provided  with  a  fluorescent  eye  piece. 

The  Cm -(iiin.— Direct  attention  specially  to  the  following  points : 

l>oes. the  line  1474  stop  short  of  the  chromosphere,  or  does  it  enter  that  forma- 
tion f  If  the  latter,  does  it  broaden  out  as  it  nears  the  sun  :'  Instruments,  .1,  <\  I). 

Are  there  any  parts  of  the  corona  which  give  a  spectrum  containing  bright  lines 
between  //,  and  14/4  '•  In  trying  to  answer  this  question  use  rather  a  wide  slit,  and, 
if  any  lines  are  found,  locate  the  part  of  the  corona  giving  them.  Instrument,  C  or  />. 

IJesides  the  well-known  bright  line  1474,  the  corona  gives  a  faint  spectrum,  which 
most  observers  have  thought  to  be  continuous.  Is  it  really  continuous,  or  is  it  crossed 
by  the  Fraunhofer  lines  ?  Look  particularly  for  />,  7-,',  1>,  and  /'',  and  if  any  of  them 
are  seen  note  in  what  part  of  the  corona.  Instrument,  (.'  or  I). 

Examine  the  dark  rifts  in  the  corona  very  carefully  to  make  sure  whether  or  not 
thev  give  any  spectrum.  Instrument,  C  or  1). 

Does  the  corona  give  any  ultra-violet  lines  ?  Instrument,  C  or  I),  provided  with 
a  fluorescent  eye  piece. 

Examine  the  brightest  parts  of  the  outer  corona  in  order  to  determine  how  far 
from  the  chromosphere  any  spectrum  is  visible,  and  note  also  what  that  spectrum  is. 
Instrument,  C  or  />. 

Examine  what  part  of  the  coronal  spectrum  is  polari/.ed.  This  may  be  done  by 
noting  the  effect  produced  upon  the  spectrum  when  a  Nicol's  prism  placed  before  the 
slit  is  rotated  ;  but  the  results  thus  obtained  will  be  complicated  by  the  polarization  due 
to  the  passage  of  the  light  through  the  dispersing  prisms,  and  to  eliminate  this  the  slit 
must  be  placed  at  different  angles  to  the  vertical,  and  upon  various  parts  of  the  corona. 
The  following  seems  a  more  satisfactory  mode  of  proceeding:  If  it  is  borne  in  mind 
that  the  polarization  of  the  corona  is  partly  radial  to  the  sun  and  partly  in  the  plane 
passing  through  the  zenith,  the  sun,  and  the  observer's  eye,  it  will  be  evident  that  the 
parts  of  the  corona  cut  by  a  vertical  circle  passing  through  the  sun's  center  are  polar- 
i/.ed only  in  the  plane  of  that  circle.  Hence,  if  the  >lit  of  the  spectroscope  is  placed 
in  that  plane,  and  the  image  of  the  sun  is  allowed  to  fall  upon  it  in  such  a  wav  that 
the  sun's  center  coincides  with  the  axis  of  the  slit;  and  if  the  Nicol's  prism  is  placed 
with  its  principal  section  at  right  angles  to,  and  immediately  before',  or  immediatelv 
behind,  the  slit,  then  it  is  evident  that  all  verticaliv  polari/.ed  light  will  be  excluded, 
and  the  spectrum  visible  will  be  due  solely  to  the  unpolari/.ed  light  of  the  corona. 

Examine  the  thermal  properties  of  the  coronal  spectrum.  Eor  this  purpose  a 
delicate  thermoelectric  pile  ami  galvanometer  will  be  required:  and  as  glass  is  coin 


23 

paratively  opaque  to  heat  waves,  a  reflecting1  telescope  should  be  employed,  the  lenses 
of  the  spectroscope  should  be  of  rock  salt,  and  the  dispersion  should  be  obtained,  either 
by  means  of  a  rock-salt  prism,  or  by  means  of  a  reflecting  diffraction  grating. 

As  totality  approaches,  observe  with  an  integrating  spectroscope  what  rays  fade 
away  and  then  brighten ;  and  observe  particularly  whether  all  the  Fraunhofer  lines 
are  reversed  at  the  instant  of  totality  During  totality  note  the  intensity  of  (7  relatively 
to  F,  and  the  intensity  of  1474  relatively  to  F,  C,  7>;!,  and  b.  Bear  in  mind  that  the 
relative  intensities  may  change  during  totality,  and  pay  attention  to  that  point!  Watch 
for  the  reversal  of  the  Fraunhofer  lines  at  the  close  of  totality.  Instrument,  7?  or  E. 

Examine  the  appearance  of  the  eclipsed  sun  with  a  meteor  spectroscope,  having 
neither  collimator  nor  slit.  So  far  as  the  corona  is  monochromatic  it  will  be  distinctly 
seen  notwithstanding  the  prism,  while  those  portions  of  it  which  shine  only  by  re- 
flected sunlight  will  be  indistinct,  their  light  being  dispersed.  The  same  object  may 
be  attained  to  some  extent  by  merely  looking  at  the  corona  through  an  ordinary 
prism,  or  through  a  direct-vision  combination.  Instrument,  F. 

Photof/rajtliy. — The  duration  of  totality  is  so  brief  that  it  is  exceedingly  desirable 
to  photograph  the  spectra  then  seen,  in  order  that  they  may  afterward  be  examined  at 
leisure.  Attempts  to  accomplish  this  have  been  made  with  two  very  different  kinds  of 
apparatus,  but  thus  far  with  only  moderate  success.  One  variety  of  the  apparatus, 
known  as  the  prismatic  camera,,  belongs  to  the  class  of  slitless  spectroscopes,  and  con- 
sists of  a  photographic  camera  before  whose  objective  a  prism  is  placed,  the  whole 
being  equatorially  mounted  and  driven  by  clock-work.  The  adjustment  of  the  prism 
to  minimum  deviation  is  effected  by  the  aid  of  a  collimator  temporarily  placed  before 
it ;  but  on  some  accounts  it  would  be  advantageous  to  dispense  with  the  prism  entirely, 
and  to  use  in  its  stead  a  diffraction  grating  placed  behind  the  objective.  The  photo- 
graphs produced  by  this  instrument  exhibit  a  dispersed  series  of  spectral  images  of 
the  corona  and  prominences,  each  image  being  due  to  light  of  a  definite  degree  of 
refrangibility.  The  other  variety  of  apparatus  consists  of  a  spectroscope  of  the  ordi- 
nary form,  except  that  it  is  provided  with  a  small  camera  instead  of  a  reading  tele- 
scope, the  whole  being  either  attached  to  an  equatorially-mounted  reflector  of  short 
focus,  or  else  so  arranged  upon  a  table  that  an  image  of  the  sun  may  be  thrown  upon 
its  slit  by  means  of  a  suitable  heliostat  and  lens.  With  the  prismatic  camera,  pictures 
were  obtained  during  the  eclipse  of  April  6,  1875  ;  but  thus  far  it  is  believed  that  no 
results  have  been  got  from  the  other  form  of  apparatus.  The  reason  for  this  is  not 
apparent,  because  the  brightness  of  the  image  depends  mainly  upon  the  intensity  ratio 
of  the  lens  by  which  it  is  depicted  upon  the  sensitive  plate,  and  there  is  no  reason  why 
this  ratio  may  not  be  the  same  in  the  two  forms  of  apparatus.  Theoretically,  the  slit- 
less  form  should  work  slightly  the  quickest,  because  it  has  fewest  optical  surfaces  ;  but, 
as  an  offset  to  this,  there  is  great  difficulty  in  determining  the  wave-lengths  of  the 
.somewhat  complicated  series  of  spectral  images  which  it  produces. 

A  careful  search  in  the  library  of  the  Naval  Observatory  failed  to  reveal  any 
data  upon  which  to  found  an  estimate  of  the  value  of  the  exposure  constant  for  the 
1474  line.  Under  these  circumstances,  if  spectroscopes  with  slits  are  used  it  will 
probably  be  best  to  expose  a  single  plate  during  the  whole  three  minutes  of  totality; 
and  as  prisms  give  brighter  spectra  than  diffraction  gratings,  the  former  will  be  pref- 
erable for  any  moderate  degree  of  dispersion ;  unless,  indeed,  the  object  is  to  photo- 


24 

irraph  tin-  ultra  violet  r;ivs.  If  prismatic  cameras  are  employed,  the  exposure  constant 
should  probably  IK-  from  0.234  to  0.469 — at  least,  these  were  the  values  used  l>y  the 
Kuglish  c\])edition  to  Siaiu  in  their  work  on  the  eclipse  of  April  6,  1875. 

SECTION  VII.—  I'OLAUISCOPIC  OBSERVATIONS. 

( Hiservations  of  the  polarization  of  the  corona  wore  first  undertaken  at  a  time 
when  it  was  still  uncertain  whether  the  corona  was  a  true  solar  phenomenon,  or  was 
in  some  way  produced  l>y  the  earth's  atmosphere ;  and  their  primary  object  was  to 
decide  between  these  two  hypotheses.  Since  then  the  spectroscope  and  photography 
have  placed  the  solar  origin  of  the  corona  beyond  doubt;  but  the  polariscopic  obser- 
vations have  proved  so  delicate  that  the  results  hitherto  obtained  from  them  cannot  be 
regarded  as  final.  Besides,  the  details  of  the  polarization,  if  sufficiently  well  defined, 
may  reveal  something  of  the  condition  of  the  matter  emitting  the  coronal  light;  and 
if  photography  can  be  substituted  for  eye-observations,  information  may  even  be  ob- 
tained concerning  regions  further  from  the  sun's  surface  than  any  of  which  we  have 
at  present  cognizance. 

Dfsrrijition  of  fiifttntiiicittfi. — The  polariscope  has  assumed  many  forms,  but  those 
best  adapted  to  the  investigation  now  under  consideration  may  be  briefly  described, 
as  follows : 

Iiixti-nn«'iit  A. — A  Savart  polariscope,  consisting  of  a  compound  quart/  plate  and 
a  Nicol's  prism  or  tourmaline:  preferably  the  latter.  This  instrument  indicates  the 
presence  of  polarized  light  by  exhibiting  a  series  of  bands,  either  parallel  or  perpen- 
dicular to  the  plane  of  polarization  ;  and  it  is  so  delicate  that  these  bands  become 
visible  whenever  the  polarized  light  amounts  to  so  much  as  one  per  cent,  of  the  total 
illumination.  To  discriminate  between  the  plane  of  polarization  and  a  plane  perpen- 
dicular to  it,  we  must  know  whether  the  central  band  is  black  or  white,  and  as  that  is 
a  difficult  question  when  the  bands  are  faint,  it  will  be  desirable  to  make  a  small  addi- 
tion to  the  apparatus.  The  Savart  polariscope  being  attached  to  the  eye-piece  of  a 
telescope  in  the  usual  way,  a  light-colored  tourmaline  should  be  inserted  in  the  eve- 
piece  in  such  a  position  as  to  cut,  off  a  small  segment  of  the  field  of  view:  the  axis  of 
the  tourmaline  being  parallel  to  the  edge  or  chord  of  the  segment,  and  the  bands  being 
perpendicular  to  that  chord.  Of  course  the  tourmaline  must  be  so  placed  as  to  be  dis- 
tinctly visible  through  the  eye-piece,  and  it  must  also  maintain  its  position  relatively 
to  the  Savart  when  the  eye-piece  is  rotated.  With  this  arrangement,  the  plane  of  po- 
larization will  be  determined  by  noting  whether  the  bands  continue  across  the  tour- 
maline or  break  off  at  its  edge 

liixti-iniii'iit  I'>.—\  pair  of  l.abinet's  quartz  compensating  wedges,  placed  at  the 
common  focus  of  the  objective  and  eye-piece  of  a  telescope,  the  eve-piece  being  pro- 
vided with  an  analyzer  consisting  of  a  Nicol's  prism  or  tourmaline.  This  instrument 
indicates  the  presence  of  polarized  light  by  exhibiting  a  series  of  bands,  which  make 
an  angle  of  forty-live  degrees  with  the  plane  of  polarization.  It  nearly  equals  the 

Savart  in  delicacy,  and   excels  it  in  the   facility  it   affords  for  determining  the  plai f 

polarization,  because  the  position  of  the  middle  band  can  be  marked  upon  the  quartz, 
and  then  a  glance  suffices  to  show  whether  it  is  black  or  \\hite. 

/    •l,',iiiii-iit  ('. — A  biquartz,  arranged  in  the  same  manner  as  the  I'.abinet's  wedge.- 


25 

just  described.  This  instrument  indicates  the  presence  of  polarized  light  by  the  colora- 
tion of  the  quartz ;  which,  however,  does  not  become  perceptible  until  the  polarized 
light  amounts  to  from  5  to  15  per  cent,  of  the  total  illumination.  If  the  biquartz 
is  turned  till  its  two  halves  are  colored  alike,  both  being  of  the  purple  hue  known  as 
the  sensitive  tint,  or  tint  of  passage,  then  the  line  of  junction  of  the  two  pieces  will  be 
in  the  plane  of  polarization,  and  as  a  very  slight  rotation  suffices  to  destroy  the  uni- 
formity of  the  tint,  this  position  can  be  found  with  great  exactness.  Thus  it  appears 
that  a  biquartz  is  inferior  to  the  instruments  A  or  B  in  sensitiveness  to  feebly  polar- 
ized light,  but  superior  to  them  in  the  accuracy  with  which  it  will  determine  the  plane 
of  polarization. 

Instrument  D. — An  Arago  polariscope,  which  consists  either  of  a  double-image 
prism  and  a  plate  of  quartz  cut  parallel  to  the  axis,  or  of  a  double-image  prism  and 
a  plate  of  selenite.  It  will  be  most  convenient  to  mount  the  prism  and  plate  of  quartz, 
or  selenite,  in  a  brass  cell,  which  can  either  be  used  alone,  or  can  be  inserted  in  one 
end  of  a  tube  about  a  foot  long  which  contains  at  its  opposite  extremity  a  diaphragm 
of  such  size  that  the  two  images  of  it,  seen  on  looking  through  the  polariscope,  may 
appear  just  in  contact  with  each  other.  This  instrument  indicates  the  presence  of 
polarized  light  by  the  coloration  of  the  field  of  view,  and  has  about  the  same  degree 
of  sensitiveness  as  the  biquartz,  but  it  is  not  well  adapted  to  the  exact  determination 
of  the  plane  of  polarization. 

Instrument  E. — An  achromatic  double-image  prism. 

Instrument  F. — A  Nicol's  prism,  or  a  tourmaline.  These  instruments,  as  well  as  E, 
are  specially  valuable  when  the  light  is  faint,  but  they  will  scarcely  indicate  the  pres- 
ence of  polarization  unless  its  amount  is  at  least  10  per  cent,  of  the  total  illumination. 

A  Polarimctcr  will  be  necessary  to  determine  the  percentage  of  polarized  light 
present.  It  should  consist  of  four  plates  of  glass,  turning  upon  an  axis  lying  in  a 
plane  parallel  to  their  surfaces  and  at  right  angles  to  their  length,  and  carrying  an  index 
moving  over  a  graduated  circle  arranged  to  show  the  angle  through  which  the  plates 
are  turned.  The  object  whose  light  is  to  be  tested  is  viewed  through  the  polarimeter 
with  a  Savart's  polariscope,  so  set  that  the  bands  seen  in  it  are  parallel  to  the  axis  of 
rotation  of  the  plates  ;  then  the  whole  instrument  is  turned  until  the  bands  are  per- 
pendicular to  the  plane  of  polarization  of  the  light  to  be  tested;  and,  finally,  the  plates 
are  rotated  until  the  bands  disappear.  When  this  happens,  the  amount  of  polarization 
present  is  just  equal  to  that  produced  by  the  plates,  the  amount  of  which  is  known 
from  the  reading  of  the  index. 

With  the  exception  of  D  and  E,  which  are  best  used  in  the  hand,  all  the  above- 
described  polariscopes  should  be  attached  to  telescopes  provided  with  positive  eye- 
pieces magnifying  about  twenty  diameters,  and  giving  pencils  of  light  sufficiently  large 
to  fill  the  pupil  of  the  eve  completely.  Before  using  the  telescopes  their  lenses  should 
be  examined  by  the  aid  of  polarized  light,  to  make  sure  that  they  are  free  from  strain, 
and  after  putting  the  instruments  together  again  special  care  must  be  taken  to  see  that 
all  reflections  are  stopped  out  in  the  tubes.  Any  neglect  of  these  precautions  may 
lead  to  serious  errors. 

Points  to  fee  investigated.— On  the  whole,'  the  observations  hitherto  made  seem  to 
lead  to  the  conclusion  that  the  corona  is  strongly  polarized  in  planes  passing  through 
4 


26 


the  sun's  center ;  or,  in  other  words,  radially ;  anil  that  upon  this  radial  polarization 
there  is  superposed  a  much  feebler  vertical  polarization,  which  is  nearly  uniform  over 
tin-  whole  corona.  The  radial  polarization  is  probably  due  to  reflections  taking  place 
within  a  few  hundred  thousand  miles  of  the  sun's  surface,  and  Professor  Pickering-  lias 
shown  that  if  it  is  produced  in  accordance  with  Fresnel's  theory  it  should  be  strongest 
in  the  outer  corona,  and  should  diminish  to  nothing  close  to  the  sun's  limb.  The  ver- 
tical polarization  is  probably  due  to  the  action  of  our  own  atmosphere. 

Theoretically  there  should  be  no  difference  between  the  indications  of  an  Arago 
polariscope  with  a  quartz  plate  and  one  with  a  selenite  plate;  and  upon  ordinary 
objects  the  results  given  by  these  two  forms  of  the  instrument  aime  certainly  identical; 
but  Professor  Pickering  thinks  that  his  observations  upon  the  eclipses  of  August,  1869, 
and  December,  1870,  show  that  there  is  some  peculiarity  about  the  corona  which 
causes  its  light  to  behave  differently  toward  a  quartz  plate  from  what  it  does  toward 
a  selenite  one. 

From  these  considerations,  it  will  be  gathered  that  the  questions  demanding  special 
attention  are  the  following : 

1.  Is  the  light  of  the  corona  polarized  radially  or  vertically,  or  in  both  these  ways? 

2.  If  radial  polarization  exists,  does  it  extend  with  unabated  intensity  down  to 
the  sun's  limb,  or  does  it  continually  diminish,  and  finally  vanish,  in  passing  from  the 
outer  corona  inward  to  the  chromosphere  ? 

3.  What  is  the  percentage  of  polarized  light  present  ? 

4.  In  examining  the  corona  with  an  Arago  polariscope,  do  the  results  given  by 
an  instrument  with  a  quartz  plate  differ  from  those  given  by  an  instrument  with  a 
selenite  plate? 

MctJiods  of  Observation. — If  a  beam  of  light  polarized  in  a  vertical  plane  is  exam- 
ined through  any  of  the  polariscopes  described  above,  the  appearances  presenting 
themselves  as  the  instrument  is  rotated  will  be  as  indicated  in  the  following  table : 


1<2 

11 

II 

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w    5- 

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g  "^     W5 

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.s   c  "a 

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uiBbcg.         S0S2P 

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HI 

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lali 

§H  -S  J 

^    o    u    i; 
DO  V  r«  ^ 

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0 

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PQ 

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0 

Maximum. 

Disappear. 

Violet. 

Red. 
Green. 

Maximum. 
Minimum. 

Maximum. 

45 

Disappear. 

Maximum. 

Green,  Red. 

Colorless. 

Equal. 

90 

Maximum. 

Disappear. 

Yellow. 

Red,  Green. 

Max.,  Min. 

Minimum. 

«35 

Disappear. 

Maximum. 

Green,  Red. 

Colorless. 

Equal. 

1  80 

Maximum. 

Disappear. 

Violet. 

Green. 
Red. 

Minimum. 

Maximum. 

Maximum. 

225 

Disappear. 

Maximum. 

Red,  Green. 

Colorless. 

Equal. 

270 

Maximum. 

Disappear. 

Yellow. 

Green,  Red. 

Min.,  Max. 

Minimum. 

3'5 

Disappear. 

Maximum. 

Red,  Green. 

Colorless. 

Equal. 

27 

Of  course  the  changes  indicated  in  the  table  take  place,  not  suddenly,  but 
gradually  and  continuously,  as  the  polariscope  is  rotated.  With  the  Savart  and  Babinet 
instruments,  if  the  bands  are  black  centered  at  the  first  and  third  maxima  they  will 
be  white  centered  at  the  second  and  fourth  maxima,  or  vice  versa.  In  describing  the 
action  of  the  biquartz  and  Arago  instruments  the  colors  red  and  green  are  mentioned; 
but  it  must  be  clearly  understood  that  the  two  complementary  tints  actually  given  by 
any  instrument  are  determined  solely  by  the  thickness  of  the  quartz  or  selenite  plate 
employed. 

At  first  sight  the  table  seems  to  indicate  that  no  two  polariscopes  are  governed 
by  the  same  law ;  but  such  a  conclusion  would  be  a  mistake.  They  all  exhibit  two 
perfectly  distinct  test  phenomena,  or  reactions ;  as  for  example,  bands  with  a  white 
center  and  bands  with  a  black  center,  or  a  given  tint  and  its  complementary,  or  a 
maximum  and  a  minimum  of  light ;  and  if  in  any  given  position  of  the  principal  sec- 
tion of  the  polariscope  one  of  these  reactions  is  at  a  maximum,  then  the  same  reac- 
tion will  also  be  at  a  maximum  when  the  position  of  the  polariscope  is  1 80°  different, 
while  for  positions  90°  or  270°  different  the  other  reaction  will  be  at  a  maximum. 

Now,  supposing  the  eclipsed  sun  to  be  viewed  through  a  telescope  armed  with  a 
polariscope,  let  us  apply  the  principles  just  enunciated  to  the  solution  of  the  first  of 
the  questions  proposed  above.  The  corona  will  be  brought  to  the  center  of  the  field 
of  view,  and  the  polariscope  will  be  turned  until  it  exhibits  a  maximum  of  one  of  the 
reactions.  Then,  if  the  polarization  is  vertical,  this  reaction  will  be  exhibited  over  the 
whole  corona ;  but  if  the  polarization  is  radical,  the  corona  will  appear  divided  into 
four  quadrants,  the  first  and  third  of  which  will  exhibit  one  of  the  reactions  while  the 
second  and  fourth  will  exhibit  the  other.  Next,  the  polariscope  will  be  slowly  rotated, 
while  the  corona  is  kept  steadily  in  the  center  of  the  field  of  view.  Then,  if  the 
polarization  is  vertical,  the  first  reaction  will  gradually  fade  out  and  be  replaced  by 
the  second,  which  in  its  turn  will  be  again  replaced  by  the  first,  and  so  on,  as  indicated 
in  the  table  above ;  but  if  the  polarization  is  radial,  the  two  reactions  exhibited  in  the 
alternate  quadrants  of  the  corona  will  not  undergo  any  change,  except  that  they  will 
rotate  with  the  polariscope. 

The  observations  necessary  for  the  solution  of  the  second  question  proposed 
above  may  be  made  with  almost  any  polariscope  by  noting  whether  or  not  the  reactions 
just  described  continue  undiminished  quite  down  to  the  moon.  It  is  probable,  how- 
ever, that  the  results  obtained  from  a  biquartz  will  be  most  satisfactory.  It  should  be 
set  so  that  the  normal  to  the  moon's  limb  makes  an  angle  with  the  line  of  junction  of 
its  two  halves  sufficiently  large  to  cause  their  colors  to  contrast  vividly.  Then  the 
observation  will  consist  in  noting  whether  this  contrast  continues  with  unabated  force 
all  the  way  to  the  moon's  limb,  or  whether  it  gradually  diminishes,  and  finally 
fades  out. 

The  amount  of  polarized  light  present  in  the  corona  can  only  be  determined  by 
measurements  with  the  polarimeter,  the  method  of  using  which  has  been  already 
described. 

In  dealing  with  the  fourth  question  proposed  above,  it  is  desirable  that  the  two 
Arago  polariscopes  employed  should  give  as  nearly  as  possible  the  same  colors.  The 


28 

method  of  proce  durewill  be  as  follows :  Firstly,  the  corona  will  be  examined  through 
one  of  the  polariscopefl,  furnished  with  its  tube  and  diaphragm,  and  the  reactions,  both 
upon  the  corona  and  upon  the  sky,  will  be  noted;  secondly,  the  tube  and  diaphragm 
will  be  removed  from  tin-  brass  cell  containing  the  prism  and  plate,  and  the  corona 
will  be  again  observed.  In  this  case,  the  images  of  two  portions  of  the  sky,  distant 
from  each  other  about  t\vo  and  a  half  degrees,  will  be  superposed,  thus  eliminating 
their  polarization,  and  therefore  it  will  only  be  necessary  to  note  the  reaction  upon 
the  corona  itself;  thirdly,  all  the  observations  will  be  repeated  with  the  other  polari- 
scope. 

In  order  to  familiarize  themselves  with  the  appearances  likely  to  be  presented 
during  the  eclipse,  intending  observers  will  do  well  to  practice  beforehand  upon  an 
artificial  corona  radially  polarized,  such  as  has  been  described  by  Professor  Picker- 
ing in  the  United  States  Coast  Survey  Report  for  1870,  page  167.  The  polariscopes 
which  seem  likely  to  give  the  best  results  are  Babinet's  compensating  wedges  and  the 
biquartz.* 

Photography. — Among  the  various  forms  of  polariscopes  the  Nicol's  and  don  hi  e- 
image  prism  seem  almost  the  only  ones  available  under  the  peculiar  conditions  which 
surround  attempts  to  photograph  the  corona.  If  a  Nicol  is  employed,  its  aperture  will 
probably  be  comparatively  small,  and  on  that  account  it  should  be  placed  behind  the 
photographic  objective,  and  near  its  focal  point;  but  if  a  double-image  prism  is  used, 
its  aperture  should  be  sufficient  to  permit  its  being  placed  immediately  before  the 
objective.  On  the  whole,  the  simplest  form  of  photographic  polariscope,  and  that  most 
likely  to  yield  satisfactory  results,  seems  to  be  an  ordinary  camera,  provided  with  a 
photographic  objective  having  an  intensity  ratio  of  about  one-eighth  and  a  focal  dis- 
tance of  about  twelve  inches,  immediately  before  which  is  mounted  a  double-image 
prism  with  a  clear  aperture  equal  to  that  of  the  objective.  With  this  apparatus  wet 
collodion  plates  of  ordinary  sensitiveness  should  be  exposed  about  thirty  seconds,  and 
after  each  exposure  the  prism  should  be  rotated  thirty  degrees,  the  motion  being  in 
the  same  direction  as  that  of  the  hands  of  a  watch. 

SECTION  VIIL— PHOTOMETRIC  OBSERVATIONS. 

- 

As  there  is  now  reason  for  suspecting  great  variability  both  in  the  size  and  in  the 
brightness  of  tlie  corona,  exact  photometric  observations  during  total  solar  eclipses 
become  of  much  importance ;  and  in  making  them,  attention  may  be  directed  either 
to  the  general  illumination  of  the  atmosphere,  or  to  the  quantity  of  light  emitted  by 
the  corona. 

For  investigating  the  first  of  these  points  the  apparatus  devised  by  Prof.  J.  II. 
Eastman,  and  used  by  him  in  observing  the  eclipse  of  August  7,  1869,  will  be  found 

•For  farther  details  consult  the  following:  Report  on  Observations  of  the  Total  Solar  Eclipse  of  December  22,  1870. 
By  Prof.  E.  C.  Pickering.  U.  8.  Coast  Survey  Report  for'iS/o,  p.  165.  List  of  Observations  on  tin-  I'ol:iri/.ation  of  the 
Corona.  By  Prof.  E.G.  Pickering.  Journal  of  the  Franklin  Institute,  1871,  vol.  61,  p.  58.  Instructions  for  Observers  at 
tin-  English  GoviTimiriit  Eclipse  Expedition,  1871.  Nature,  1871,  vol.  5,  p.  18.  Suggestions  to  Observers  of  t  lie  Solar 
Eclipse  of  DcctMnber  next.  By  A.  C.  Ranyard.  Nature,  1871,  vol.  4,  p.  327.  English  Government  Eclipse  Expedition, 
1875.  Instructions  to  Observers.  Nature,  1875,  vol.  n,  p.  352. 


29 

/ 

very  satisfactory.*  On  that  occasion  the  light  during  totality  proved  about  equal  to 
that  on  a  clear  moonless  evening,  at  the  time  Avhen  the  sun  has  sunk  so  far  below  the 
horizon  that  third  magnitude  stars  are  just  becoming  easily  visible. 

For  measuring  directly  the  amount  of  light  emitted  by  the  corona,  some  form  of 
Bunsen's  photometer  will  be  required,  and  probably  that  adopted  by  Professor  Pick- 
ering will  be  found  as  convenient  as  any.f  It  consists  of  a  box  9  inches  wide,  1 8 
inches  high,  and  6  feet  long,  within  which  a  lighted  candle  can  be  moved  backward 
and  forward  by  means  of  a  long  projecting  rod.  One  end  of  the  box  is  covered  by  a 
piece  of  thin  white  unruled  writing  paper,  in  the  center  of  which  is  a  greased  spot 
about  half  an  inch  in  diameter.  The  observation  consists  in  pointing  the  box  toward 
the  corona,  so  that  the  rays  of  the  latter  may  fall  squarely  upon  one  side  of  the  paper 
while  those  of  the  candle  fall  upon  the  other;  moving  the  candle  by  means  of  the  rod 
until  the  greased  spot  disappears ;  and  then  marking  upon  the  rod  by  a  stroke  of  a 
lead-pencil  the  distance  between  the  candle  and  paper.  A  number  of  disappearances 
should  be  observed,  and  after  totality  is  over  the  distances  between  the  marks  and  the 
candle  must  be  measured  and  recorded.  During  the  eclipse  of  December  22,  1870, 
Professor  Pickering  found  that,  on  the  average,  the  candle  had  to  be  18.5  inches  from 
the  paper  to  make  the  spot  disappear,  but  the  observations  were  much  embarrassed  by 
flying  clouds. 

Owing  to  the  difference  of  color  between  daylight  and  candle-light,  no  adjust- 
ment of  the  candle  will  suffice  to  make  the  greased  spot  disappear  completely ;  it  can 
only  be  reduced  to  a  minimum  of  visibility.!  Care  must  be  taken  to  have  the  interior 
of  the  photometer-box  painted  dead  black  with  a  mixture  of  lamp-black,  shell-lac,  and 
alcohol.  The  candle  must  be  of  wax  or  sperm,  of  the  kind  known  as  six  to  the 
pound,  and  experiments  must  be  made  to  determine  exactly  how  many  grains  it  burns 
per  hour ;  or,  better,  the  candle  may  be  sent  to  the  Naval  Observatory  and  these  expe- 
riments will  be  made  there. 

Persons  who  are  inexperienced  in  photometry  should  have  some  practice  previous 
to  the  eclipse.  The  necessary  apparatus  will  be,  two  lighted  candles  placed  three  or  four 
feet  apart,  and  a  sheet  of  thin  white  writing-paper,  having  in  its  center  a  spot  half  an 
inch  in  diameter,  rendered  transparent  by  the  application  of  a  little  grease.  The  practice 
will  consist  in  holding  the  paper  in,  and  at  right  angles  to,  the  line  joining  the  two 
candles,  and  then  moving  it  backward  or  forward  until  the  greased  spot  disappears, 
which  it  will  do  when  the  illumination  on  the  two  sides  of  the  paper  is  exactly  equal. 
When  this  experiment  has  become  quite  familiar,  the  photometer  which  is  to  be  em- 
ployed during  the  eclipse  should  be  taken  into  a  room  dimly  lighted  by  daylight,  and 
the  intensity  of  the  illumination  should  be  repeatedly  measured  by  making  the  spot 
disappear  as  completely  as  possible,  and  then  noting  the  interval  between  the  candle 
and  the  paper.  Excellent  practice  may  also  be  had  by  measuring  the  intensity  of 
twilight  in  the  early  evening. 

*  Reports  on  Observations  of  the  Total  Eclipse  of  the  Suu,  August  7,  1869,  Appendix  II  to  the  Washington  Observa- 
tions for  1867,  p.  100. 

t  See  United  States  Coast  Survey  Report  for  1870,  p.  172. 

t  The  greased  spot  can  be  made  to  disappear  completely  by  using  paper  which  is  blue  on  one  side  and  white_on  the 
other ;  but  this  cannot  be  recommended,  because  it  introduces  serious  theorectical  difficulties. 


30 

• 

SECTION  IX.— THERMO-ELECTRIC  OBSERVATIONS. 

The  principal  points  to  be  investigated  are  the  total  amount  of  heat  emitted  by 
the  corona  and  chromosphere,  and  the  relative  temperatures  of  the  prominences  and  of 
different  parts  of  the  corona.  The  galvanometer  employed  should  be  very  sensitive, 
and  it  will  be  advisable  to  have  some  small  resistances  which  can  be  inserted  in  the 
circuit  to  reduce  the  deflections  of  the  needle  in  case  the  currents  given  by  the  pile 
prove  unmanageably  strong. 

For  investigating  the  first  of  the  points  mentioned  above,  an  ordinary  thermo- 
pile, provided  with  the  usual  conical  reflector,  will  be  the  best  instrument ;  and  it 
should  be  used  by  pointing  it  to  the  corona  without  the  intervention  of  any  condensing 
lens  or  mirror  whatever.  After  measuring  the  heat  in  that  way,  its  so-called  quality 
may  be  tested  by  noting  the  effects  produced  by  the  interposition  of  screens  of  glass 
or  other  transparent  material  of  various  known  thicknesses. 

For  investigating  the  relative  temperature  of  the  prominences,  and  of  different 
parts  of  the  corona,  a  thermopile  placed  in  the  focus  of  a  telescope,  which  should  be 
a  reflector  if  possible,  will  be  required;  and  as  we  are  ignorant  of  the  degree  of  heat 
to  be  expected,  the  pile  should  not  be  too  delicate.  A  very  suitable  one  can  be  made 
by  taking  a  piece  of  iron  wire  about  six  inches  long,  soldering  to  each  of  its  extremi- 
ties pieces  of  german-silver  wire,  and  then  bending  the  whole  into  the  form  of  an  M, 
the  solderings  being  at  the  points  forming  the  top  of  the  M.  The  wire  should  not  be 
more  than  one  or  two  hundredths  of  an  inch  in  diameter,  and  the  whole  should  be 
so  mounted  in  a  piece  of  cork  that,  the  junctions  may  project  only  a  short  distance 
above  its  surface.  The  currents  given  by  this  pile  will  be  due  solely  to  the  differences 
of  temperature  of  its  two  junctions,  and,  consequently,  the  galvanometer  will  be 
insensible  to  any  flow  of  heat  which  affects  both  junctions  alike.  The  mode  of  using 
the  instrument  will  be  to  place  one  of  its  junctions  upon  the  moon,  or  upon  a  promi- 
nence, and  the  other  upon  the  point  whose  relative  temperature  is  to  be  measured. 
One  of  the  most  important  points  to  be  ascertained  is  how  much  more  heat  comes 
from  the  prominences  than  from  the  neighboring  portions  of  the  corona. 

As  soon  as  possible  after  the  totality  is  ended,  the  constants  of  the  apparatus 
should  be  most  carefully  determined,  so  as  to  furnish  the  means  of  converting  the  gal- 
vanometer readings  into  ordinary  thermometric  degrees. 


Plate  1. 


fioftont 


f 


• 


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V 


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